The present invention relates to a noise filter incorporated in, for example, a switching power source or a DC-DC converter.
In recent years, a reduction in the size, weight and production cost of the office automation (OA) equipment has advanced, and the significance of the above-described types of power sources in the OA equipment has grown, thus increasing a demand for a reduction in the size of such a power source or a noise filter incorporated in the power source.
Noise filters, whose size reduction has been demanded, must have a higher attenuation capability in order to cope with higher frequencies.
Generally, the characteristics required for the soft magnetic material for use in a magnetic core of a noise filter are as follows:
(1) High saturation magnetization
(2) High magnetic permeability
(3) Low coercive force, and
(4) Thin shape which can easily be formed.
In view of the above, various alloys have been studied in the course of developing such soft magnetic alloys for use as in a magnetic core of a noise filter. Particularly, alloys exhibiting higher saturation magnetization and higher permeability have been studied in order to achieve reduction in the size of the noise filter and an increase in the frequencies that the noise filter can cope with.
Conventional materials for use in the magnetic core of a noise filter are crystalline alloys, such as Fe--Al--Si alloy Permalloy or silicon steel, and Fe-based or Co-based amorphous alloys.
However, Fe--Al--Si alloy suffers from a disadvantage in that the saturation magnetization thereof is as low as about 11 kG, although it exhibits excellent soft magnetic characteristics. Permalloy, which has an alloy composition exhibiting excellent soft magnetic characteristics, also has a saturation magnetization as low as about 8 kG. Silicon steel (Fe--Si alloys) has inferior soft magnetic characteristics, although they have a high saturation magnetization.
Co-based amorphous alloys have an insufficient saturation magnetization, which is about 10 kG, although they exhibit excellent soft magnetic characteristics. Fe-based amorphous alloys tend to exhibit insufficient soft magnetic characteristics, although they have a high saturation magnetization, which is 15 kG or above. Further, amorphous alloys are insufficient in terms of the heat stability and this deficiency may cause a problem.
Thus, it is conventionally difficult to provide a material exhibiting both high saturation magnetization and excellent soft magnetic characteristics. This in turn makes it difficult to provide a noise filter exhibiting sufficient attenuation characteristics.